For some decades there has been a search for processes for the production of cellulose moulded bodies as a substitute for the viscose process, which is widely employed. An alternative which is interesting for its reduced environmental impact among other reasons, is to dissolve cellulose without derivatisation in an organic solvent and extrude from this solution moulded bodies such as fibers, films and membranes. Fibres thus extruded have been accorded by BISFA (The International Bureau for the Standardization of man made fibers) the generic name Lyocell. By an organic solvent, BISFA means a mixture of an organic chemical and water.
It has turned out that as an organic solvent, a mixture of a tertiary amine-oxide and water is particularly appropriate for the production of cellulose moulded bodies. As the amine-oxide, primarily N-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides are described e.g. in EP-A- 0 553 070. A process for the production of mouldable cellulose solutions is known e.g. from EP-A- 0 356 419. The production of cellulose moulded bodies using tertiary amine-oxides generally is referred to as amine-oxide process.
U.S. Pat. No. 4,246,221 describes an amine-oxide process for the production of cellulose solutions which are spun into filaments in a forming tool such as a spinneret and afterwards passed through a precipitation bath, wherein the cellulose is precipitated and water-containing, swollen filaments are obtained. These filaments may be processed to cellulose fibers and staple fibers in the conventional way, i.e. by washing and post-treatment. It is known that the cellulose fibers produced from amineoxide solutions according to the dry/wet spinning process have, in contrast to natural, crimped cellulose fibers such as cotton, an unlobed, round cross-section. When they are processed to yarns and plane fibre assemblies, the round cross-section and the relatively smooth surface may cause problems, as described e.g. in EP-A- 0 574 870. According to this patent application, these problems include a deficient adhesion of the fibers to each other when the spinning fibre is spun to yarns, an insufficient cover of the filament yarns and insufficient slippage resistance of the plane fibre assemblies produced from this fibre and filament yarns. To solve these problems, the above patent application proposes to extrude the amine-oxide solution through spinning holes having a cross-section which is not circular but shaped, for example Y-shaped. Thus, the Lyocell fibers get a Y-shaped section.
In Chemical Fibers International (CFI), volume 45, February 1995, pages 27 and 30, the microscopic illustration of four cellulose fibers all produced according to the amine-oxide process is shown. It is interesting that these fibers are not identical, although all of them are produced according to the amine-oxide process. The differences between the four fibers can be seen even under the microscope. In the literature cited it is not indicated how those skilled in the art may produce the different cellulose fibers, in other words no information is given to those skilled in the art how it is possible to make each of the fibers look different.
In Textilia Europe 6/94, pages 6ff, also a cellulose fibre produced according to the amine-oxide process is described, and again those skilled in the art are not given any clues about the details of the production. Based on other information, it can be gathered from this literature that the cellulose fibre, the production of which is not indicated, has a permanent crimp, but no more detailed information given as to what is meant by this and how the fibre may be crimped.
Crimped fibers are advantageous various reasons for, particularly for processing them into staple fibers. For instance, it is easier to card the fibers, since a certain adhesion of the fibers among each other is required to produce a card sliver. A crimped fibre has a higher sliver adhesion than a non-crimped fibre, and thus it is possible to increase the carding rate.
In the prior art, so-called crimp processes whereby fibers may be crimped are known. However, a crimping thus achieved is mostly lost after carding and even moreso after spinning to yarns, and is not found any more in the textile fabric. Crimping if present, would give a bulky, soft feel to the textile fabric.
From WO 94/28220 and WO 94/27903, a process whereby Lyocell fibers may be crimped in a mechanical way is known. According to this process, the freshly produced, tow-shaped filaments first are passed through a number of washing baths to remove the solvent. Then the tow is dried at approximately 165.degree. C. and introduced in a dry state into a pipe-shaped device, wherein the filament tow is creased and thus some kind of crimping is achieved. Additionally, the crimped fibre is treated with hot, dry vapour and afterwards cut to a staple fibre. These fibers have the drawback that their production requires a complex arrangement, since a separate device for crimping is required, and that crimping is achieved by creasing the fibers. Moreover, it has been shown that crimping carried out in a mechanical way according to that known process is lost again for the fibre after some further post-processing steps.
It is the object of the invention to provide a process for the production of a new Lyocell fibre which may be processed to yarns and fabrics in an easier way than the conventional Lyocell fibre. The new fibre is not to be produced by means of mechanical crimping according to WO 94/28220 or WO 94/27903. Neither is the fibre to be produced using spinnerets exhibiting spinning holes which have a noncircular cross-section. Rather, the Lyocell fibre produced according to the invention is to be produced using conventional spinnerets having spinning holes which exhibit a circular cross-section.